The experimental crystal structure of CsPbCO3F consists of alternate CsF and PbCO3 layers.

We explore the effect of spin-orbit interaction (SOI) on the electronic and optical properties of CsPbCOF using the full potential linear augmented plane wave method with the density functional theory (DFT) approach. CsPbCOF is known for its high powder second harmonic generation (SHG) coefficient (13.4 times (d = 0.39 pm V) that of KHPO (KDP)). Calculations are done for many exchange correlation (XC) potentials. After the inclusion of SOI, the calculated Tran-Blaha modified Becke-Johnson (TB-mBJ) band gap of 5.58 eV reduces to 4.45 eV in agreement with the experimental value. This is due to the splitting of Pb p-states. Importantly, the occurrence of a band gap along the H-A direction (indirect) transforms to the H-H (direct) high symmetry points/direction in the first Brillouin zone. We noticed a large anisotropy in the calculated complex dielectric function, absorption, and refractive index spectra. The calculated static birefringence of 0.1049 and 0.1057 (with SOI) is found to be higher than that of the other carbonate fluorides. From the Born effective charge (BEC) analysis we notice that the Cs atom shows a negative contribution to birefringence whereas Pb, C, and F atoms show a positive contribution. In addition, we have also calculated the nonlinear optical χ(-2ω;ω,ω) dispersion of a CsPbCOF single crystal. We found that d = d = 4.35 pm V at 1064 nm, which is 11.2 times higher than d of KDP. The origin of the highly nonlinear optical susceptibility dispersion of CsPbCOF is explained. Overall, our results are in agreement with experiments and it is obvious from the present study that CsPbCOF is a direct band gap, large second harmonic generation, and good phase matchable NLO crystal in the ultraviolet region.

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An ab initio study on the impact of hydrostatic pressure and strain on the electronic properties of an unexplored class of ternary Zintl phases KZnX (X = P, As, Sb) is reported. Density functional theory (DFT) based studies revealed that all the three materials are direct band gap semiconductors under ambient conditions. We have theoretically demonstrated that KZnX can be driven into different metallic phases under pressure. In contrast, by applying strain the compounds can be realized as topological insulators. This is confirmed by the observed non-trivial topological character in the electronic band structure of the present ternary systems. For the precise determination of low energy band topology, the Tran Blaha modified Becke-Johnson (TBmBJ) exchange potential was used by incorporating spin-orbit coupling. The concomitant change of electronic band shapes as a function of pressure indicates a semi-metallic nature in KZnX (X = P, As, Sb) at 30 GPa, 21 GPa and 11 GPa respectively. Based on an analysis of the parity eigenvalues, we anticipate that a band inversion occurs between the Zn-s and X-p states, thus demonstrating a weak topological behaviour in semi-metallic states. Also, a weak non-trivial topologically insulating phase is realized in strained KZnAs (18%) and KZnSb (10%) which appears to be due to overlapping of the Zn-s and X-p orbitals. The calculated surface spectral functions further validate the non-triviality of strained KZnX (X = As, Sb), whereas strained KZnP is found to be a trivial insulator. We confirm the topological behaviour of these materials by calculating topological surface states and defining a Z topological invariant. Our work based on sophisticated first-principles calculations highlights that both pressure and strain can trigger topological phases in non-symmorphic trivial band insulators even with a weak spin orbit interaction. This study paves the way for realizing semi-metallic and topological insulating states in non-symmorphic ternary semiconductors, which have not been experimentally demonstrated so far.

We report a detailed study on structural, vibrational, born effective charge (BEC), electronic and optical properties of the alkali metal perchlorates, MClO4(M  =  Li, Na, K, Rb, Cs) based on Density functional theory. The ground state calculations are done using plane wave pseudopotential method by including dispersion corrected method for more accurate prediction of structural and vibrational frequencies. The calculated lattice parameters and bond lengths are consistent with the experimental values. Further, detailed interpretation of the zone centered vibrational modes yields good concurrence between the experimental and calculated values. There is a decrease in wavelength with an increase in frequency (blue shift) from Li  →  Na  →  K  →  Rb  →  Cs. The obtained BEC shows the mixed covalent-ionic character of the compounds. The electronic and optical properties are calculated using the full potential linearized augmented plane wave method by TB-mBJ potential. The TB-mBJ band structure shows indirect band gap with O-2p states dominating in the valence band. In spite of anisotropic structure, alkali metal perchlorates are found to possess optical isotropy.

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The knowledge of mechanical, vibrational, electronic properties, and origin of polarizability of a noncentrosymmetric explosive materials plays a crucial role in understanding the phase transition mechanism and stand-off detection of high energy material (HEM) residues on the surfaces and interfaces. In the present study, we have explored the role of crystal structure and chemical composition in predicting the structural, dynamical, electronic, and optical properties of two newly synthesized noncentrosymmetric green primary explosives, potassium 4,4′-bis(dinitromethyl)-3,3′-azofurazanate (K 2 BDAF) and potassium 1,1′-dinitramino-5,5′-bistetrazolate (K 2 DNABT), using density functional theory simulations. The calculated structural and mechanical properties suggest that K 2 BDAF and K 2 DNABT are mechanically stable and possess lower bulk modulus values [K 2 BDAF (18.91 GPa) < K 2 DNABT (22.4 GPa)] than toxic lead azide Pb(N 3 ) 2 (26 GPa). The Born effective charge (BEC) and vibrational, thermodynamic, and phonon dispersion studies reveals that both crystals are dynamically stable and that K 2 BDAF is found to be more stable, more polarizable, and easily detectable than K 2 DNABT. The TB-mBJ electronic band structure study reveals that K 2 BDAF is an indirect band gap (along R → V high symmetry points) material with a gap of 2.37 eV < K 2 DNABT (3.57 eV, direct). The density of states and charge density plots show that both crystals possess mixed covalent and ionic bonding nature, and these results are consistent with BEC analysis. The nitrogen, oxygen and carbon p states are found to be dominant near the Fermi level in the valence band. From the optical properties study, it is found that the studied explosive materials have the lowest dielectric constant compared with the well-known 4-N,N-dimethylamino-4-N-methylstilbazolium tosylate (DAST) crystal, i.e., K 2 DNABT (2.68) < K 2 BDAF (2.91) < DAST (5.2), and show birefringence similar to (K 2 BDAF static value, 0.32; at 532 nm, 0.58), (K 2 DNABT static value, 0.25; at 532 nm, 0.27) that of DAST (0.39, 0.55, 0.64) and 4-N,Ndimethylamino-4′-N′-methylstilbazolium 2,4,6-trimethylbenzenesulfonate (DSTMS) (0.45, 0.63) NLO crystals. For both the studied explosives, the absorption is found to be strong in the [010] direction and follows the relation α(K 2 BDAF) < α(K 2 DNABT). The analysis of differences in BEC reveals that from K 2 DNABT to K 2 BDAF, the positive contribution of atoms to total birefringence increases as follows: K1 (0.09559), O1 (0.69236), O2 (0.77962), N1 (1.41018), N2 (0.92394), N3 (0.27415), N4 (0.74264), and N5 (0.84502). The negative contribution from C1 is increased by −1.29582. Over all, our results are in agreement with the experimentally reported sensitivity trend, and the reasons behind the sensitivity differences of K 2 DNABT and K 2 BDAF are explored.

Inorganic metal azides M(N3)2 (M  =  Sr, Ba) and metal nitrates M(NO3)2 (M  =  Sr, Ba) are interesting materials due to their wide range of industrial usefulness as gas generators, pyrotechnics, photo graphic materials and explosives. In this work, we explore the mechanical, vibrational (infrared, phonon dispersion), Born effective charge (BEC) and thermodynamic properties of these materials to understand the sensitivity correlation studies using plane wave pseudopotential method. As these materials are layered with crucial non bonding interactions, the generalized gradient approximation with Tkatchenko–Scheffler (for Sr(N3)2) and Ortmann–Bechstedt–Schmidt (for Ba(N3)2) dispersion correction methods are adopted to compute accurate ground state properties with norm-conserving pseudopotentials. The calculated lattice parameters, unit cell volumes, bond lengths are well reproduced with 1% deviation when compared to the experimental and previously reported theoretical data. The mechanical (single crystal, poly-crystalline elastic constants) property correlations corroborate with the experimental impact sensitivity trend. The vibrational, phonon dispersion spectra’s, BEC’s, thermodynamic properties calculated with density functional perturbative theory approach provide better conclusions about the dynamical stability and polarization (optical sensitivity) trends. From the BEC results we propose that M(NO3)2 (M  =  Sr, Ba) materials may find various optical applications too. Overall, we tried to explain the crucial reasons for the differences in energetic properties of the studied materials.